Leave it on overnight and boot the kids off! Or if you want to be cruel and your router supports it, restrict certain websites and prioritise traffic 😉 Though in fairness, Steam is quite sensible about downloads so it will pick up again next time you start up the Steam program and work away in the background. Think bittorrent in the way it does things so it will adjust accordingly.
BTW, I think if the games are there on your old PC in the steam directory if you have downloaded already so you could drag and drop them re-linking them. I recall doing it when changing drives on my pc (not exactly easy but do-able) but google it and guaranteed someone will have done it already.
